As set forth in the aforementioned US Patent Application Publication No. US 2006/0291527 A1 (“'527 Publication”), it is advantageous to heat fluids, particularly liquids such as water for use as domestic hot water using a “tankless” heating device. A tankless heating device is intended to heat the fluid as it flows from a source to a point of use. A tankless heater does not rely on a stored reservoir of preheated liquid, but instead is designed with sufficient capacity to heat the liquid to the desired temperature, even as the liquid flows through the heater at a rate equal to the maximum expected demand. For example, if a tankless heater is intended to provide hot water to shower in a home, the heater is designed with sufficient capacity to heat water at the lowest expected incoming temperature to the highest desired shower temperature at the maximum flow rate of the shower.
As disclosed in the '527 Publication, one form of fluid heater particularly suitable for liquids such as domestic water heating is a direct electric resistance liquid heater. In a direct electric resistance liquid heater, electrical power is applied between electrodes immersed in the liquid to be heated so that current flows through the liquid itself and power is converted into heat due to the electrical resistance of the liquid itself. As also disclosed in the '527 Publication, such a heater can be arranged with multiple electrodes defining numerous channels for liquid flow. The control system for such a heater may be arranged to connect and disconnect different ones of the electrodes to a power supply. The electrodes and associated elements of the heater can be arranged so that connection of different sets of the electrodes to the electrical power supply connection provides different levels of current passing through the liquid. These levels most preferably include a step-wise progression between zero current when none of the electrodes are connected and a maximum current when all of the electrodes are connected. As disclosed in the '527 Publication, this progression desirably has substantially uniform ratios between the currents of adjacent steps of the progression having non-zero current levels. As explained in the '527 Publication, heaters having such a set of possible current levels can provide progressive control of liquid temperature despite wide variations in incoming liquid temperature, desired outgoing liquid temperature, flow rate, and resistivity of the liquid. The desired step-wise progression desirably includes numerous steps as, for example, 60 or more steps or different current levels for fluid of a given resistivity. Most preferably, the steps are arranged so that the maximum ratio between the current levels in any two adjacent steps of the progression having non-zero currents is no more than about 1.22:1, and preferably no more than about 1.1:1, and so that the greatest difference between levels of current in any two adjacent steps of the progression is no greater than about 10% of the maximum current for the given level of fluid resistivity.
Because the heat is evolved within the liquid itself, such a heater can provide essentially instantaneous heating of the liquid flowing through it. Moreover, the heater can be controlled by simply connecting and disconnecting different ones of the electrodes to the power supply, allowing use of switching elements such as conventional relays or, more preferably, solid-state semiconductor switching elements such as triacs and field effect transistors. The preferred semiconductor switching elements can be brought to a conducting or “closed” state in which they have very low electrical resistance, or a substantially non-conducting state in which they have extremely high, almost infinite resistance and conduct essentially no current, and thus act as an open switch. Thus, the semiconductor elements themselves dissipate very little power, even though substantial electrical currents flow through them when they are in their closed states.
The heater disclosed in the '527 Publication includes a temperature sensor arranged to sense the temperature of the heated liquid near a controller responsive to the signal from the temperature sensor for controlling the switching elements, and thereby controlling the power applied by the heater to the flowing liquid. The preferred temperature sensor taught in the '527 Publication includes a “thermally conductive temperature sensing plate” which is “placed as close as practicable to the end of the heating chamber and perpendicular to the flow of liquids such that the liquid leaving the heating chamber must pass through the perforations of the temperature sensing plate,” and also includes a “semiconductor junction based temperature sensor” mounted to the plate. As set forth in the '527 Publication, however, such an arrangement suffers from “thermal lag or delay” between changes in temperature of the heated liquid and the signal output from the thermal sensor because of the thermal resistance of the thermal plate and packaging of the thermal sensor and the “thermal mass” of these components. To compensate for this, the control system includes a signal conditioner circuit which creates a signal which represents “the rate of change of the temperature as measured by the temperature sensor,” and this signal is summed with the signal representing the temperature itself. While this arrangement provides satisfactory operation, further improvement would be desirable.